The present invention is directed to copolymer compositions useful in the preparation of fuel tank sealants. More particularly, the present invention relates to condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer compositions and the use of polyalkoxysilylorganic compounds as cross-linkers to facilitate the condensation cure of poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymers.
As shown by Smith, U.S. Pat. No. 3,109,826, bis(alkoxysilyl) hydrocarbons, such as 1,2-bis-(triethoxysilyl)ethane, can be used as cross-linkers in combination with a metal salt to effect the neutral condensation cure of hydroxy end-blocked polydiorganosiloxanes. However, the resulting cured silicone compositions have been found to be problematic as aircraft fuel tank sealants, as they do not have the required solvent or fuel resistance. It is known that fluorosilicones, for example, made by polymerizing tris[(trifluoropropyl)methyl]cyclosiloxane, can provide excellent fuel resistance. However, fluorosilicones often do not meet the wide temperature stability requirements needed in aircraft sealants, such as temperatures in a range between xe2x88x9254xc2x0 C. and 177xc2x0 C. over an extended period of time. In addition, fluorosilicones are subject to depolymerization which can result in the formation of low molecular weight cyclics.
In an effort to enhance the thermal stability of fluorosilicones, non-siloxane groups, such as p-silphenylene, have been inserted into the polyfluorosiloxane backbone, as shown by Grassie and Beattie, xe2x80x9cThe Thermal Degradation of Polysiloxanes: Part 7xe2x80x9d, Polymer Degradation and Stabilization 8:177-193 (1984). It is also reported by Dvornic and Lenz, Macromolecules, 25, 3769 (1992), that copolymers having a glass transition temperature (Tg) of xe2x88x9251xc2x0 C. can be made by reacting methyl(3,3,3-trifluoropropyl)silanediol and 1,4-bis(dimethylhydroxysilyl)benzene.
While fluorosilicones having improved thermal stability have been made by inserting non-siloxane groups, such as p-silphenylene into the polyfluorosiloxane backbone, such copolymers have been found to have a glass transition temperature which does not satisfy the minimum xe2x88x9254xc2x0 C. Tg flexibility requirements of aircraft fuel tank sealants.
Experience also has shown that in addition to being sensitive to depolymerization, condensation curable fluorosilicone compositions often suffer from an incomplete cure using a conventional curing catalyst, such as a tin salt, and a standard neutral condensation curable cross-linker, for example a polyalkoxysilane. One possible explanation, as discussed by Fujiki, U.S. Pat. No. 5,236,997, is that the steric hindrance or electronic effects of bulky terminal trifluoropropyl groups inhibit crosslinking of the network.
Accordingly, depolymerization resistant silicone base copolymers which could be compounded to a fuel resistant condensation curable silicone composition convertible to the elastomeric state upon cure exhibiting stability over an operable temperature in a range between about xe2x88x9254xc2x0 C. or below and at least about 177xc2x0 C. over an extended period of time are constantly being sought which also exhibit low temperature flexibility, in addition to high temperature stability.
The present invention provides a condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer exhibiting a glass transition temperature not exceeding about xe2x88x9254xc2x0 C.
A further embodiment of the present invention provides a method for making a poly(fluoroorgano)siloxane-poly(silarylene)siloxane copolymer comprising effecting reaction between a bis(diorganohydroxysilyl)arylene and a poly(fluoroalkylorgano)cyclopolysiloxane
In yet a further embodiment of the present invention, there is provided a neutral condensation curable poly(fluoroorgano)siloxy-poly(silarylene)siloxane block copolymer sealant composition comprising
(a) a poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer,
(b) a cross-linker, and
(c) a condensation catalyst.
In yet another embodiment of the present invention, there is provided a method for making a neutral condensation curable poly(fluoroorgano)siloxy-poly(silarylene)siloxane block copolymer sealant composition which comprises
(a) effecting reaction between bis(diorganohydroxysilyl)arylene and poly(fluoroalkylorgano)cyclopolysiloxane to form a condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer,
(b) shearing the copolymer, and
(c) blending a cross-linker and a condensation catalyst with the copolymer to form a sealant.